1. Field of the Invention
The present invention pertains generally to a video monitor used in various computer systems and, more particularly, relates to a video monitor adjustment system for adjusting a displayed picture under the control of a microprocessor.
2. Discussion of Background
A conventionally known adjustment system for a video monitor comprising a cathode ray tube (CRT) employs a microprocessor built in the video monitor (Japanese Unexamined Patent Publication No. 3-35287).
For optimum adjustment of the picture quality of a video monitor, it is necessary to set operating parameters for a video amplifier circuit, a vertical deflection circuit, a horizontal deflection circuit, etc. to optimum values. If a microprocessor is utilized, it is possible to complete the process of video monitor adjustment by transmitting those operating parameters to specific elements, which need to be adjusted, through a digital-to-analog (D/A) converter. The microprocessor designed to perform such adjustment functions is provided with a nonvolatile memory for retaining these operating parameters. In addition, there is provided an operator input unit incorporating operating switches and a group of indicator lamps which makes it possible to select adjustment items and to increase or decrease specific operating parameters through the microprocessor.
Although it is possible to automatically adjust such operating parameters in accordance with operation of different computer systems in the prior art technology, an operator is required to be completely aware of and skilled in every process of adjustment since the aforementioned adjustment system is not provided with any message display functions except for the indicator lamps incorporated in the accompanying operator input unit. Furthermore, the adjustment system inevitably requires a rather complicated adjustment procedure.
With technological advances in personal computers, video monitors used as display terminals of computer systems are required to provide interchangeability. Specifically, if video signals which are input to a specific video monitor do not conform to standardized specifications, video signal timing may not match the video monitor and displayed data would not necessarily appear at the center of its screen. To prevent this kind of problems, a technique has thus far been proposed that makes it possible to present video signals at the center of the CRT screen regardless of video signal specifications (e.g., Japanese Unexamined Patent Publication No. 5-27698).
According to this technique, there is provided a control circuit comprising a central processing unit (CPU) for measuring input video signal timing. Specifically, the control circuit measures a period of time from a leading edge of a horizontal synchronizing signal to a leading edge of a video signal and that from a leading edge of a vertical synchronizing signal to a leading edge of the video signal. (These periods of time are hereinafter referred to as horizontal display start time and vertical display start time, respectively.) The control circuit can adjust phases of a horizontal timing signal in a horizontal deflection circuit and of a vertical timing signal in a vertical deflection circuit in accordance with the measurement results so that the horizontal and vertical positions of a displayed picture correctly align with the center of the CRT screen. With this control circuit, it becomes possible to automatically position the displayed data at the center of the screen. In other words, the control circuit can automate horizontal position adjustment and vertical position adjustment.
However, the aforementioned control circuit has a problem in that its time measuring accuracy and its picture positioning accuracy are insufficient. This is because the control circuit does not have any function to adjust the size of the displayed picture and it simply measures the horizontal and vertical display start times as the input video signal timing by using a timer built in the CPU. Another problem of this control circuit is that it is applicable to limited types of input video signals since its operation is based on the assumption that the horizontal and vertical deflection frequencies of an input video signal are held constant.